It is important to know and be able to regulate the torque to be applied when installing a combination nut and bolt in order to connect two or more structural elements. Too little clamping force, usually applied in the form of a torque, can leave the connection loose so that it may succumb to an early fatigue failure. Too much clamping force can result in damage to the elements being joined. Even if the elements being joined can physically withstand over-tightening, such over-tightening may cause damage to one of the fasteners resulting in stripped threads, and the like.
One means to regulate torque, and therefore clamping force, has been to use a torque wrench. This tool is ideal in situations where the fasteners being torqued are few in number or where the operator could take the time to precisely tighten each fastener. However, in aerospace applications, tens and hundreds of thousands of fasteners must be installed in a relatively short time. Therefore, the ability to measure the torque of each bolt and nut combination during installation is simply not a viable option.
Torque limiters have been incorporated into pneumatic drivers to save time and increase efficiency. However, such torque limiters have resulted in both higher cost and questionable reliability. To reduce cost and improve reliability, nut and bolt sets, such as that described in U.S. Pat. No. 5,145,300 issued to John Wallace on Sep. 8, 1992 and known commercially as Ellipto-Lok, have self-locking features designed integrally therewithin.
However, aerospace fasteners must not only be cost effective, they must also be securely locked to prevent loosening under severe vibratory conditions. An inadequately locked fastener is of major concern, because loose fasteners have been known to be ingested into jet engines and cause severe damage thereto. The self-locking feature of the Ellipto-Lok fastener is therefore advantageous, but its reliability depends on the dimensional integrity of its driving socket or wrench which is known to be susceptible to degradation through wear. That is, the socket or wrench used to install the nut and bolt set may become worn with use, changing its clearances and tolerances. In some extreme cases, the socket may be so worn that it begins to act unpredictably on the nuts to produce unacceptably low installation torque and, consequently, create preload with loss of locking integrity.
Accordingly, it would be desirable to have available a torque measuring device that is adapted to provide a reliable means for measuring the changing output torque generated by an installation tool over time so that the life of the tool can be carefully monitored to insure that proper torquing forces will be applied during the installation of nut and bolt sets.